The present invention is directed towards an improved cooling system for a gas turbine. More particularly, the present invention is directed towards an improved cooling system which employs flow-resistance devices to meter coolant into a plurality of platform and air foil coolant channels located in the buckets of the gas turbine.
The cooling system of the present invention is utilized in connection with a gas turbine of the type including a turbine disk mounted on a shaft rotatably supported in a casing and a plurality of turbine buckets extending radially outward from the disk. Each of the buckets includes a root portion mounted in the disk, a shank portion extending radially outward from the root portion to a platform portion, and an air foil extending radially outward from the platform portion. During operation, the buckets receive a driving force from hot fluids moving in a direction generally parallel to the axis of the shaft and convert this driving force to rotational motion which is transmitted to the shaft via the turbine disk. As the result of the relatively high temperatures of the hot fluid, a significant amount of heat is transferred to the turbine buckets. In order to remove this heat from the bucket structure, the prior art has developed a large variety of open-liquid cooling systems. Exemplary of such systems are U.S. Pat. No. 3,658,439, issued to Kydd; U.S. Pat. No. 3,804,551, issued to Moore; U.S. Pat. No. 4,017,210, issued to Darrow; and U.S. application Ser. No. 044,660, filed June 1, 1979, in the names of C. M. Grondahl and M. R. Germain, now U.S. Pat. No. 4,244,676. the disclosure of which is incorporated herein by reference.
Open circuit liquid cooling systems are particularly important because they make it feasible to increase the turbine inlet temperature to an operating range of from 2500.degree. F. to at least 3500.degree. F., thereby obtaining an increase in power output ranging from about 100%-200% and an increase in thermal efficiency ranging to as high as 50%. A primary requirement of open circuit liquid cooling systems is that the liquid coolant be evenly distributed to the several platform and air foil coolant channels formed in the bucket. Such a distribution is difficult to obtain as a result of the extremely high bucket tip speeds employed, resulting in centrifugal fields of the order of 25,000 G.
To obtain an even flow of coolant liquid throughout the several coolant channels, the prior art systems, as exemplified by U.S. Pat. Nos. 3,804,551 and 4,017,210, supra, utilize weir structures which meter the amount of coolant liquid supplied to each individual channel from pools of coolant liquid formed in the platform portion of the bucket. Particularly, these systems introduced liquid coolant into each end of a trough formed in the platform portion of the bucket such that liquid coolant flows in a direction parallel to the axis of rotation of the turbine disk from each end of the trough. The liquid coolant flows over the top of an elongated weir which performs the metering for each channel.
In order to perform satisfactorily, it is critical that the tops of these weirs be parallel to the axis of rotation of the turbine within a tolerance of several mils. If this relationship is not maintained, all of the coolant liquid will flow over the low end of the weir and, consequently, some of the coolant channels formed in the platform and air foil of the bucket will be starved for coolant.
In an effort to overcome the foregoing problem, the invention described in U.S. Pat. No. 4,244,676 utilizes V-shaped notched weirs which are less sensitive to variations in the orientation in the metering channels than the prior art weirs. While this invention represents an improvement over the prior art weir structures, all weir metering devices depend upon a uniform depth of water above the crest of each weir to ensure an equal supply of cooling water to the individual bucket cooling channels. While the V-shaped notched weirs make the accuracy of flow metering less sensitive to manufacturing tolerances and in-service distortion, it is still affected by waves on the surface of the water in the reservoir supplying the weirs. Such waves have been found to occur as a result of oscillations in the flow rate of water to the metering device and may also result from rotor vibrations.